Pollutant-free low temperature combustion process having carbonaceous fuel suspended in alkaline aqueous solution

Abstract

A continuous process for the combustion of carbonaceous fuels under conditions such that oxides of nitrogen are not formed and oxides of sulfur and particles of ash are effectively prevented from contaminating the gaseous products released to the atmosphere. Fuel is charged as a slurry in alkaline aqueous solution and contacted with combustion air so that the catalytic properties of both water and alkali operate to permit rapid and complete combustion at unusually low temperatures. Useful heat is extracted from the heated mixture. At the low combustion temperatures, sulfur in the fuel oxidizes to the trioxide which dissolves completely in the alkaline liquid phase which also retains particles of ash and unburned fuel.

Having described my invention I claim: 
     
       1. A process for converting the heating value of a carbonaceous fuel to useful energy comprising the steps of: grinding the fuel to form fuel particles;   mixing the fuel particles with an alkaline aqueous fluid to form a fuel slurry;   contacting the fuel slurry with air in a reaction zone the temperature and pressure of which are such as to cause a combustible portion of the fuel particles to burn while a portion of the aqueous fluid remains in liquid phase; and   extracting useful heat from the reaction zone.   
     
     
       2. A process as in claim 1 in which the step of contacting the fuel slurry with air includes the steps of: charging the fuel slurry to a reaction zone in which a mixed slurry of combustible and non-combustible particles comprises a continuous phase; and   distributing the air in the lower part of the reaction zone so that it bubbles upward as a gaseous phase through the continuous slurry phase.   
     
     
       3. A process as in claim 2 in which the contact between the mixed slurry and the air is enhanced by mechanical agitation. 
     
     
       4. A process as in claim 2 in which there are a plurality of reaction zones arranged so that the flow of the slurry phase between the reaction zones is counter-current to the flow of the gaseous phase between the reaction zones. 
     
     
       5. A process as in claim 2 in which the step of extracting useful heat includes the steps of: withdrawing a stream of the mixed slurry from the reaction zone to a zone of reduced pressure, causing some of the water contained therein to vaporize, forming steam;   separating the steam from unvaporized slurry; and   returning a portion of the unvaporized slurry to the reaction zone.   
     
     
       6. A process as in claim 2 in which the step of extracting useful heat includes the steps of: withdrawing a stream of the mixed slurry from the reaction zone to a zone of reduced pressure, causing some of the water contained therein to vaporize, forming primary steam;   separating the primary steam from unvaporized slurry;   subjecting the unvaporized slurry to a second reduction in pressure causing additional water contained therein to vaporize, forming secondary steam;   separating the secondary steam from slurry remaining unvaporized; and   returning a portion of the slurry remaining unvaporized to the reaction zone.   
     
     
       7. A process as in claim 2 in which the step of extracting useful heat from the reaction zone includes transferring the useful heat through heat exchange surface to a heat transfer medium. 
     
     
       8. A process as in claim 7 in which the heat transfer medium is boiling water. 
     
     
       9. A process as in claim 5 and including the additional steps of: cooling a portion of the unvaporized slurry;   reducing the pressure of the cooled slurry to a pressure near atmospheric;   separating the cooled and depressurized slurry into liquid and solid components; and   utilizing a portion of the liquid component as an ingredient of the alkaline aqueous fluid.   
     
     
       10. A process as in claim 6 and including the additional steps of: cooling a portion of the slurry remaining unvaporized;   reducing the pressure of the cooled slurry to a pressure near atmospheric;   separating the cooled and depressurized slurry into liquid and solid components; and   utilizing a portion of the liquid component as an ingredient of the alkaline aqueous fluid.   
     
     
       11. A process as in claim 1 in which the step of contacting the fuel slurry with air includes the steps of: mixing the fuel slurry with the air;   causing the mixture to flow through an elongated reaction zone in which the temperature is regulated by transfer of heat through heat exchange surface to a heat transfer medium; and   separating the effluent of the reaction zone into slurry and gaseous phases.   
     
     
       12. A process as in claim 11 in which the heat transfer medium is boiling water. 
     
     
       13. A process as in claim 1 in which the step of contacting the fuel slurry with air includes the steps of: mixing the fuel slurry with partially exhausted air;   causing the mixture of fuel slurry and partially exhausted air to flow through a first elongated reaction zone in which the temperature is regulated by transfer of heat through heat exchange surface to a first heat transfer medium;   separating the effluent of the first reaction zone into slurry and gaseous phases;   mixing the slurry from the first reaction zone with the air;   causing the mixture of the first reaction zone slurry and air to flow through a second elongated reaction zone in which the temperature is regulated by transfer of heat through heat exchange surface to a second heat transfer medium; and   separating the effluent of the second reaction zone into a slurry phase and a gaseous phase comprising the partially exhausted air.   
     
     
       14. A process as in claim 13 in which the first heat transfer medium and the second heat transfer medium are boiling water. 
     
     
       15. A process as in claim 11 and including the additional steps of: cooling the separated slurry phase;   reducing the pressure of the cooled slurry phase to a pressure near atmospheric;   separating the cooled and depressurized slurry phase into liquid and solid components; and   utilizing a portion of the liquid component as an ingredient of the alkaline aqueous fluid.   
     
     
       16. A process as in claim 13 and including the additional steps of: cooling the separated second reaction zone slurry phase;   reducing the pressure of the cooled slurry phase to a pressure near atmospheric;   separating the cooled and depressurized slurry phase into liquid and solid components; and   utilizing a portion of the liquid component as an ingredient of the alkaline aqueous fluid.   
     
     
       17. A process as in claim 11 and which includes the additional step of: recycling a portion of the separated slurry phase to the inlet of the elongated reaction zone.   
     
     
       18. A process as in claim 13 and which includes the additional step of: recycling a portion of the separated second reaction zone slurry phase to the inlet of the first elongated reaction zone.   
     
     
       19. A process as in claim 13 and which includes the additional step of: recycling a portion of the separated second reaction zone slurry phase to the inlet of the second elongated reaction zone.   
     
     
       20. A process as in claim 1 in which the step of contacting the fuel slurry with air is carried out at a temperature between 550° and 705.4° F. and at a pressure between 1000 and 10,000 pounds per square inch.